1. Fields of the Invention
The present invention relates generally to an apparatus and method for reducing workpiece handling overhead relative to active workpiece processing time in a manufacturing process, and, more particularly, to reducing the relative time spent pumping a vacuum in semiconductor wafer processing chambers, venting such chambers to atmosphere, and transferring wafers to and from such chambers while increasing the relative time spent actively processing the wafers in the chambers, such as, by actively etching, stripping, and depositing the semiconductive layers of the wafers, and, even more particularly, to the process of switching common RF or microwave power supply sources alternately between dual downstream or in-chamber plasma reactors and alternately actively processing in one reactor while performing the aforesaid overhead tasks on the other reactor thereby significantly increasing the throughput of the overall machine at overall reduction in equipment cost compared to conventional dual or multiple reactor systems.
2. Discussion of Background and Prior Art
a. Generating a Plasma
The primary reason to generate a plasma is to generate an intense amount of heat in a localized area to excite atoms or molecules of gas to an elevated state. The energy can be so intense that some common compounds dissociate.
For example, CF.sub.4, a stable compound gas, can have some of the fluorine atoms stripped off to form monatomic F which is extremely reactive. In the case of oxygen, it exists in nature as O.sub.2 and not O.sub.1. When oxygen is passed through an intense RF electric field, often produced by microwave radiation, a significant percentage actually separates into monatomic oxygen, O.sub.1, and is far more reactive than O.sub.2. The monatomic oxygen is also heated to temperatures as high or higher than 1000 degrees C. when in the plasma field. Photoresist is a carbon based or organic compound. When the wafer is heated to typically 220 to 270 degrees C. and subjected to the hot monatomic oxygen, the photoresist is combusted often within 10 to 15 seconds depending on thickness and other variables. The new compounds formed are generally CO.sub.2, CO, and H.sub.2 O which are gaseous at ambient conditions. The photoresist effluent is then pumped off in the gaseous state through a vacuum pump. In etch applications, different gas volatiles are formed.
Although plasmas can be created at atmospheric pressure, they are very difficult to ignite, difficult to control, consume enormous power, generate intense UV light, and are awkward to confine. Nearly all semiconductor process applications operate at greatly reduced pressures relative to atmospheric conditions. As a general class, plasma processing is typically conducted from a low range of 1 milli-Torr to a high range of roughly 10 Torr. For example, photoresist ashing or surface cleaning and preparation pressure ranges operate from about 0.1 Torr to 2 Torr.
Examples of methods to generate a plasma at rarified pressures are capacitively coupled electrodes, parallel plate reactive ion etchers (RIE), inductively coupled plasma (ICP) methods involving a resonant RF coil, microwave cavities, and electron cyclotron resonance (ECR). Some embodiments also include magnetic fields for shaping the plasma.
b. Vacuum Processing
Semiconductor silicon wafer processes, such as, actively etching, stripping, and depositing the semiconductor layers of the wafers, are generally performed under greatly reduced atmospheric pressure conditions ranging generally from well under the 10.sup.-3 Torr regime to a few Torr in order to readily establish plasma conditions exciting the process gas with RF or microwave energy. Plasma excited process gas species have greatly elevated reaction levels to speed combination with the intended semiconductor layer to be processed. This vacuum processing contributes substantially to the high cost of semiconductor processing equipment itself (vacuum pumps alone cost about $40,000.00) and the relatively excessive amount of time (typically 65-80% of a complete cycle) spent in the overhead tasks of pumping a vacuum, venting to atmosphere, transferring wafers to and from, opening and closing doors, metering gases in and out, and the like. Moreover, each of these subsystems is typically contained in a multi-chamber machine, providing 100% redundancy at great expense in terms of equipment cost and space. Nakane U.S. Pat. No. 4,483,651 (2:10-13).
c. Cost of Semiconductor Equipment
On one hand, when examining the cost of semiconductor wafer processing equipment, one quickly realizes that relatively few system components comprise the vast majority of the total equipment costs. The process gas mass flow controllers, the RF power sources, such as, microwave and lower frequency RF power supplies, and the vacuum pumps are among the largest high cost items followed by the actual process reactor. Significant savings can be immediately achieved simply by developing a process which increases throughput by using dual, side-by-side replicas of selected critical costly items, but one of everything else.
Accordingly, it is an object of the present invention to significantly reduce the overall costs of semiconductor processing equipment, and especially atmosphere to vacuum equipment, by eliminating all redundant components of a plural setup other than the dual, side-by-side, wafer processing chambers.
d. Overhead Time
On the other hand, when examining the typical overhead tasks performed by the semiconductor wafer processing equipment, including (1) wafer transfer from the cassette to the chamber entrance and later back to the cassette; (2) chamber door opening and closing; (3) wafer exchange to and from the process chamber; (4) wafer heating or cooling preparation; and (5) pumping or venting the process reactor chamber to the desired vacuum level, or conversely, to an elevated pressure, one quickly recognizes that a significant savings can be immediately achieved by coordinating and sharing, that is, synchronously multiplexing, common material support sub-assemblies which perform overhead tasks.
As one example of the relative speed of one type of semiconductor processing machine, such as a photoresist ashing machine, the fastest photoresist, strip, dual process chamber module unit in the marketplace today runs at typically 110-130 wafers per hour.
Accordingly, it is an object of the present invention, by adding redundancy of one critical stage of a single process machine at only a modest cost increase while synchronously multiplexing the remaining components of the processing system, virtually 100% of the wafer exchange overhead, the pump/vent overhead, and the wafer temperature conditioning overhead commonly associated with preparing an environment for wafer chemical processing to commence is eliminated or masked.
e. Current Semiconductor Atmosphere to Vacuum Continuous, Synchronously Multiplexed, Wafer Transfer Systems
(1). Solo Wafers, Early Attempts.
Early attempts to solve the problem of high expense, high overhead wafer processing are seen in Uehara U.S. Pat. No. 4,149,923 in which single wafer, single processing chambers, each separately pumped, were replaced by multi-chamber, sequential processing of single wafers all in one common vacuum, and in the system by Nakane '651 referenced above in which a computer controlled a common main wafer transfer mechanism operating in parallel with plural plasma reactors each fed by its own branch shuttle wafer transfer mechanism and each having its own vacuum pump. While achieving some economies through a common vacuum and by an automatic parallel scheme, Uehara and Nakane left too much redundancy in their entire systems.
(2). Group Batch Process
Another method commonly in use reduces the pump and vent overhead time by pumping and venting an entire group of wafers in a load lock all at one time, typically a cassette of 25 or more wafers. In an early system by Blake in U.S. Pat. No. 5,019,233 plural, separately pumped, single process, chambers are serviced by a central interior substrate handling robot which services dual load lock chambers alternately loaded with 25-piece substrate batches. While one batch is being subjected to load lock evacuation providing thermal desorption, the other batch, having been previously evacuated is transferred one at a time to selected process chambers by the central interior robot. Therefore, load lock dwell times are ordered parallel to the active (coating) process, and extended load lock dwell times do not impair apparatus productivity until the batch lock dwell time exceeds the time needed for the serial processing of a full batch of individual cassettes.
There are good reasons for transferring wafers within a vacuum environment including toxic gas control, sequential processes where exposure to air affects the process, and moisture control for cold process where moisture can freeze to surfaces. Moreover, while wafer transfer time within a vacuum equalized environment is faster, nonetheless, it continues to be an appreciable percentage of the total productive wafer process time.
Thus, while Blake's system improves overall system throughput, it is far from optimal because the cost of a cassette vacuum load lock is quite expensive especially when coupled with an expensive vacuum wafer transfer robot, and it can never provide true zero overhead because of the overhead inherently required in sequential processing of the substrate stack through plural process chambers.
In a more recent attempt by Begin in U.S. Pat. No. 5,310,410, plural, individually pumped, processing chambers each for a different function serviced by an interior central robot in a sequential processing process were provided with atmospheric level processing chambers interspersed therebetween. While this system no doubt provided the claimed increase in flexibility, it did so at a great increase in equipment cost and overhead.
Finally, in an even more recent example, by Higashi in U.S. Pat. No. 5,611,861, after describing the failure of early solo wafer processing systems to achieve sufficient throughput, after describing the failure of early batch wafer processing systems to achieve sufficient uniformity and throughput, after describing the failure of early multi-chamber wafer processing systems to achieve sufficient increase in throughput because the transfer overhead time exceeded the processing time of the processing chambers and finished wafers had to wait for transfer, and, therefore, in a further effort to increase throughput in even multi-chamber processing systems, Higashi proposed putting the plural processing chambers on a carousel supported by two fixed, spaced apart, transfer stations adjacent the carousel, one for transferring unprocessed wafers into a processing chamber in hermetically sealed fashion and the other for transferring processed wafers to the outside as the carousel rotates the processing chambers sequentially into alignment with one or the other transfer station while wafer processing is taking place therebetween. Each of Higashi's processing chambers has its own high frequency power supply for its plasma reactors. Again, we see the prior art achieve increased throughput at the expense of significantly increased equipment cost with complete 100% redundancy of critical costly items in each processing chamber without significantly improving machine component efficiency or utilization.
f. Current Semiconductor Dual Chamber Alternating Power Supply System
In a semiconductor alternating dual processing chamber system by Oramir located in Israel, it is known to alternate the power source between the dual chambers in an application which uses a raster scan laser to ablate photoresist on a semiconductor wafer in an ozone ambient at atmospheric pressure. However, this feat is accomplished by merely deflecting the laser beam with a mirror alternately from one process chamber to the other. Obviously, Oramir's system is extremely expensive and slow and is not price or throughput competitive even with conventional machines on the market. Moreover, the entire process is performed at atmospheric or near atmospheric conditions, not at typical low pressure vacuum levels. Furthermore, due to the special complexities associated with deep vacuum processing and RF power switching, there is no suggestion by Oramir to provide, and, accordingly, it is an object of the present invention to provide, a switchable radio frequency power supply alternately, synchronously multiplexed with atmosphere to vacuum dual processing chambers.
It has been amply demonstrated above that there is still a long felt need for, and it is an object of the present invention to provide, a process of providing radio frequency power alternately between dual, adjacent, downstream or in-chamber plasma reactors and actively processing in one without overlapping the processing in the other while performing the aforesaid overhead tasks in the other thereby significantly increasing the throughput of the overall machine at a substantial overall reduction in equipment cost.